The present invention relates to a fixing belt adapted to move circularly and cooperate with a pressing member to form a nip region for fixing an unfixed toner image on a recording medium. The present invention also relates to a fixing apparatus equipped with such a fixing belt.
Heretofore, a so-called heating roller system has been widely used in a fixing apparatus for electrophotographic equipment. The heating roller system typically comprises a pair of rollers each having a surface coated with a non-adhesive material, and a heat source provided inside each of the rollers. A fixing operation of this system is performed by rotating the pair of rollers with applying an appropriate pressure therebetween, and passing a recording medium with a transferred or unfixed toner between the rollers. On the other hand, an alternative technique has been developed in which a belt is wound between one of the above pair of rollers and a third roller, and unfixed toner on a recording media is pressed by the surface of the belt.
By way of example of the above technique, Japanese Patent Laid-Open Publication No. 06-318001 discloses a fixing apparatus based on a belt fixing system as shown in FIG. 2 thereof. The disclosed fixing apparatus comprises a fixing roller R1, a pressing roller R2, a heating/tension roller R3, and a fixing belt B wound between the fixing roller R1 and the heating/tension roller R3. The fixing apparatus further includes a guide plate G disposed under the lower portion of the fixing belt B with leaving a certain space which defines a heating passage P between the lower portion of the heated fixing belt B and the guide plate G to allow a recording medium D to be preheated while passing therethrough. The preheating of the recording medium D having an unfixed toner image supported thereon makes it possible to set the temperature of a nip region lower, which provides an enhanced releasability between the fixing belt B and the toner, and a desirable clearness of the fixed image without offset.
With respect to the fixing apparatus based on the belt fixing system as described above, the applicant has filed a patent application (Japanese Patent Application No. 07-088357) relating to an improved technology in which an interior angle defined by a first straight line which connects the center of a fixing roller and a contact point between the lower portion of the fixing roller and a fixing belt and a second straight line which connects the contact point and the center of a pressing roller is arranged in the range of 90xc2x0 to 175xc2x0 to provide a stable and high-quality fixed image.
As with the fixing apparatus based on the conventional belt fixing system, it is necessary for the aforementioned improved belt-type fixing apparatus to employ a fixing belt having an excellent heat conductivity and high dimensional accuracy. If a belt made of a polyimide resin as a heat-resistant polymeric material is used as the fixing belt, its insufficient heat conductivity will undesirably place a limit on the increasable range of a fixing speed, and increase the time-period required for heating the fixing belt up to a given temperature in a startup operation. While a fixing belt made of metal such as nickel is excellent in heat conductivity and dimensional accuracy, the usable period of the metal fixing belt is disadvantageously limited due to its degraded strength through a heat treatment required for providing a releasing layer thereon, resulting in deficiency of practicability.
From this point of view, the applicant has proposed a technique intended to achieve a long-life metal endless-type fixing belt suitable for belt-type fixing apparatuses and provide a metal fixing belt having a high heat conductivity, high rigidity, excellent heat resistance and excellent fatigue strength. This technique has been applied for a patent and granted as Japanese Patent No. 2706432.
The above granted patent discloses a fixing belt adapted to move circularly and cooperate with a pressing member to form a nip region for thermo-compression-bonding and fixing an unfixed toner image onto a recording medium. This fixing belt comprises a substrate formed of an endless-shaped electroformed sheet which consists of a nickel-manganese alloy containing 0.05 to 0.6 mass % of manganese and has a micro-Vickers hardness of 450 to 650.
The patented technology having these features can achieve the aforementioned objects. However, it leaves to be improved in the durability under high-temperature atmosphere, and the need of a fixing belt having further improved durability still exists.
In view of the above circumstances, it is therefore an object of the present invention to provide a fixing belt capable of achieving a desirably enhanced durability even under high-temperature atmosphere.
In order to solve the above problem and achieve the object, according to a first aspect of the present invention, there is provided a fixing belt adapted to move circularly and cooperate with a pressing member to form a nip region for fixing an unfixed toner image on a recording medium. The fixing belt comprises a substrate formed of an endless-shaped electroformed sheet consisting of a nickel-carbon alloy containing 0.05 to 0.12 mass % of carbon.
According to a second aspect of the present invention, the carbon content in the fixing belt is adjusted in a range of 0.055 to 0.10 mass %.
According to a third aspect of the present invention, the carbon content in the fixing belt is adjusted in proportion to the amount of a secondary brightening agent to be added to an electrolytic bath.
According to a fourth aspect of the present invention, the secondary brightening agent used in the fixing belt is 2-butyne-1,4-diol.
According to a fifth aspect of the present invention, the carbon content in the fixing belt may be adjusted in proportion to a cathode current density to be sent to an electrolytic bath.
According to a sixth aspect of the present invention, the substrate in the fixing belt has a surface provided with a releasing layer.
Further, according to a seventh aspect of the present invention, there is provided a fixing apparatus comprising a fixing roller, a pressing roller in rotational contact with the fixing roller at a given pressure, a heating roller disposed apart from the fixing roller, a fixing belt wound around both the heating and fixing rollers in an endless manner, and a heat-generating device for heating the fixing belt to heat unfixed toner on a sheet passing through a rotational contact region between the fixing and pressing rollers, whereby when the sheet having unfixed toner on a surface thereof is passed through the rotational contact region along one direction, the unfixed toner is fixed onto the sheet. The fixing belt includes a substrate formed of an endless-shaped electroformed sheet consisting of a nickel-carbon alloy containing 0.05 to 0.12 mass % of carbon.
In the present invention, the fixing belt essentially has an endless shape. Thus, the substrate of the fixing belt is prepared by an electroforming process using a master made of stainless steel or the like as a cathode and an electrolytic bath capable of electrocrystallizing a nickel-carbon alloy. In this case, the electrolytic bath may be a conventional nickel electrolytic bath such as a sulfamic acid solution, optionally added with an addition agent such as a PH regulator, a pit inhibitor or a brightening agent. The concentration of a secondary brightening agent based on an unsaturated organic compound, an electrolytic bath temperature, a cathode current density and/or other factor are controlled to obtain an electroformed product formed of a nickel-carbon alloy containing a desired content of carbon.
After cutting away the side ends of the electroformed product, the electroformed product is separated from the master. Then, the separated product is subjected to rinse, drying, machining and other treatment or processing so as to provide a substrate having desired shape and dimension.
Further, an optional primer is applied onto the surface of the substrate, and then a releasing layer may be formed thereon to produce the fixing belt of the present invention. The releasing layer may be formed as a single layer or multilayer made of a material having heat resistance and non-adhesive, such as silicone rubber, fluorine rubber, fluororesin, or their mixture.
The fixing belt of the present invention will repeatedly receive a bending stress when it moves circularly between the fixing and heating rollers with being wound around both the rollers. Thus, it is important to provide a sufficient fatigue resistance to the fixing belt. In view of this point, heat conductivity and dimensional stability, it is preferable to use metal in the substrate. A pure nickel electrocrystallized product has a poor mechanical strength due to its rough and large crystal structure, and thereby it has been difficult to provide a sufficient durability required for a material of the fixing belt. In order to obtain an improved electrocrystallized product having a fine or densified crystal structure, it is contemplable to add a brightening agent to the electrolytic bath. This provides an improved mechanical strength of the electrocrystallized product, but cannot produce a sufficient improvement in durability.
It has been found that the embrittlement due to repetitive bending was suppressed and the high-temperature durability could be significantly improved by selectively using as the substrate an endless-shaped electroformed sheet which consists of a nickel-carbon alloy having the content of carbon in the range of 0.05 to 0.12 mass % and has a durability particularly under high-temperature atmosphere.
The fixing belt of the present invention may be obtained by forming a releasing layer made of a material having heat resistance and non-adhesive, such as silicone rubber, fluorine rubber or fluororesin, on the surface of the substrate satisfying the above requirement of the carbon content. This fixing belt has an excellent releasability for toner even with an extremely reduced amount of releasing oil such as silicone oil, which prevents the toner from attaching to and contaminating the fixing belt, and thereby provides a clear image without generating any undesirable offset. In addition, the fixing belt exhibits an excellent long-term durability with undiminished stable quality.